All the typical salts that you would be adding to sparging water will dissolve, even in hot water. You may not have mixed enough.
I did just that with my last batch...didn't stir enough to get them dissolved. I've made this batch before, so I know that the salts had dissolved before. The problem is that I have a new electronical, fancy-pants HLT and all I have to do is fill it with the proper amount of water and turn on the power. I need to slow down and stir a little more and make sure the salts are dissolved!
But as tygo said, you can always just throw them into the kettle.